PNF stretching is currently the fastest and most effective way known to increase
static-passive flexibility. PNF is an acronym for proprioceptive
neuromuscular facilitation. It is not really a type of stretching but is a
technique of combining passive stretching (see section
Passive Stretching) and isometric stretching (see section
Isometric Stretching) in order to achieve maximum static flexibility.
Actually, the term PNF stretching is itself a misnomer. PNF was initially
developed as a method of rehabilitating stroke victims. PNF refers to any of
several post-isometric relaxation stretching techniques in which a
muscle group is passively stretched, then contracts isometrically against
resistance while in the stretched position, and then is passively stretched
again through the resulting increased range of motion. PNF stretching usually
employs the use of a partner to provide resistance against the isometric
contraction and then later to passively take the joint through its increased
range of motion. It may be performed, however, without a partner, although it is
usually more effective with a partner's assistance.

Most PNF stretching techniques employ isometric agonist
contraction/relaxation where the stretched muscles are contracted
isometrically and then relaxed. Some PNF techniques also employ
isometric antagonist contraction where the antagonists of the
stretched muscles are contracted. In all cases, it is important to note that the
stretched muscle should be rested (and relaxed) for at least 20 seconds before
performing another PNF technique. The most common PNF stretching techniques are:

the hold-relax

This technique is also called the contract-relax. After assuming an
initial passive stretch, the muscle being stretched is isometrically contracted
for 7-15 seconds, after which the muscle is briefly relaxed for 2-3 seconds, and
then immediately subjected to a passive stretch which stretches the muscle even
further than the initial passive stretch. This final passive stretch is held for
10-15 seconds. The muscle is then relaxed for 20 seconds before performing
another PNF technique.

the hold-relax-contract

This technique is also called the contract-relax-contract, and the
contract-relax-antagonist-contract (or CRAC). It involves
performing two isometric contractions: first of the agonists, then, of the
antagonists. The first part is similar to the hold-relax where, after assuming
an initial passive stretch, the stretched muscle is isometrically contracted for
7-15 seconds. Then the muscle is relaxed while its antagonist immediately
performs an isometric contraction that is held for 7-15 seconds. The muscles are
then relaxed for 20 seconds before performing another PNF technique.

the hold-relax-swing

This technique (and a similar technique called the hold-relax-bounce)
actually involves the use of dynamic or ballistic stretches in conjunction with
static and isometric stretches. It is very risky, and is
successfully used only by the most advanced of athletes and dancers that have
managed to achieve a high level of control over their muscle stretch reflex (see
section The Stretch Reflex). It is similar
to the hold-relax technique except that a dynamic or ballistic stretch is
employed in place of the final passive stretch.

Notice that in the hold-relax-contract, there is no final passive stretch. It is
replaced by the antagonist-contraction which, via reciprocal inhibition (see
section Reciprocal Inhibition), serves to
relax and further stretch the muscle that was subjected to the initial passive
stretch. Because there is no final passive stretch, this PNF technique is
considered one of the safest PNF techniques to perform (it is less likely to
result in torn muscle tissue). Some people like to make the technique even more
intense by adding the final passive stretch after the second isometric
contraction. Although this can result in greater flexibility gains, it also
increases the likelihood of injury.

Even more risky are dynamic and ballistic PNF stretching techniques like the
hold-relax-swing, and the hold-relax-bounce. If you are not a professional
athlete or dancer, you probably have no business attempting either of these
techniques (the likelihood of injury is just too great). Even professionals
should not attempt these techniques without the guidance of a professional coach
or training advisor. These two techniques have the greatest potential for rapid
flexibility gains, but only when performed by people who have a sufficiently
high level of control of the stretch reflex in the muscles that are being
stretched.

Like isometric stretching (see section
Isometric Stretching), PNF stretching is also not recommended for children
and people whose bones are still growing (for the same reasons. Also like
isometric stretching, PNF stretching helps strengthen the muscles that are
contracted and therefore is good for increasing active flexibility as well as
passive flexibility. Furthermore, as with isometric stretching, PNF stretching
is very strenuous and should be performed for a given muscle group no more than
once per day (ideally, no more than once per 36 hour period).

The initial recommended procedure for PNF stretching is to perform the desired
PNF technique 3-5 times for a given muscle group (resting 20 seconds between
each repetition). However, HFLTA cites a 1987 study whose results
suggest that performing 3-5 repetitions of a PNF technique for a given muscle
group is not necessarily any more effective than performing the technique only
once. As a result, in order to decrease the amount of time taken up by your
stretching routine (without decreasing its effectiveness), HFLTA
recommends performing only one PNF technique per muscle group stretched in a
given stretching session.